1. Field of Invention
This invention relates to improvements in linear stepper motors and more particularly to such linear stepper motors having a statorbar with a slider movably mounted on the statorbar.
2. Description of the Prior Art
Known linear stepper motors usually comprises a stator bar having a plurality of stator teeth disposed at equal intervals in a longitudinal direction and a slider having a plurality of slider teeth opposed to the stator teeth with a small gap therebetween. It is important that the stator teeth and the slider teeth be kept accurately as close to each other as possible without physical contact. Therefore, a pair of flat rail portions are provided on the stator bar and a plurality of rollers are supported on the slider for engaging with the rail portions and for maintaining the fixed small gap between the stator teeth and the slider teeth during movement of the slider.
Also, it is desirable for the characteristics and the efficiency of such linear stepper motors and for the manufacturing thereof, that the stator teeth and slider teeth be formed of a high permeability magnetic material, such as a soft steel containing a low percentage of carbon, which soft steel is mild and may be readily cut or ground. It is further desirable from the view point of manufacturing cost, that the stator teeth and the rail portions be formed of one body, such as by cutting and/or grinding one surface of the stator bar made of such material.
One deficiency of such prior motors is that during the motion of such linear stepper motor, the rail portions are often subjected to large loads which are concentrated through the rollers, because an attractive magnetic force occurs between the stator teeth and the slider teeth. The concentrated load may be about ten times as large as the driving force of such linear stepper motor. Moreover, the rail portions are subjected to another irregular shock resulting from the characteristic step-by-step movement of the stepper motor. Thus, the rail portions of the stator bar are caused to become uneven by the aforementioned large load and irregular shock. Hence, the predetermined small gap between the stator teeth and the slider teeth may become reduced little-by-little, or the distance between the two may become different at different parts of the rail portions. As a result, the characteristics and efficiency of the stepper motor tend to become increasingly inferior with continued use. In the worst case, the slider teeth may even be caused to physically contact the stator teeth, when the slider is magnetically attracted toward the stator bar.
To eliminate such deficiencies, it would be better to form the rail portions of another considerably harder material and to attach same to the stator bar. But, such a solution is not feasible since it is difficult and very complicated to make such parts of the motor of such material, and to construct an entire motor unit of such material and concurrently provide for regulating accurately the gap between the stator teeth and the slider teeth and for keeping such gap constant. It is desirable that the gap between the stator teeth and the slider teeth be as small as possible and as uniform as possible, in any position, for example, between 0.03 and 0.1 millimeters (mm) during the movement of the slider.